International Patent Application No. WO91/03145 (Kearney & Trecker) shows a hexapod machine tool comprising two supports or platforms and six extensible, powered legs or rams, each connected between the two supports or platforms. A tool on one of the platforms is movable relative to a workpiece on the other platform. Various desired positions and orientations between the tool and the workpiece can be achieved by extending the legs by differing amounts.
In such machines, the position and orientation at any instant is monitored by means of a plurality of length measuring transducers, e.g. one transducer associated with each leg. WO91/03145 discloses the use of six instrument arms, separate from the legs, each instrument arm containing a length measuring transducer. One possible type of length measuring transducer is a laser interferometer in each instrument arm. Taking six length measurements from the six instrument arms enables the position and orientation of one platform relative to the other to be determined. In practice, such length measurements are used in a feedback loop to the powered legs, to servo the movable platform precisely to the position and orientation required.
The instrument arms disclosed in WO91/03134 require a universally pivotable bearing at each end, to connect the arm to the respective platform. Therefore, if there are any inaccuracies such as backlash in these bearings, the measured lengths will be inaccurate and there will be a corresponding inaccuracy in the positioning and orientation of one platform relative to the other.